The invention relates generally to semiconductor circuit design and, more particularly, to a method and apparatus for controlled overdriving on a sense amplifying circuit.
Digital logic requires input and output data signals to meet defined voltage levels to be interpreted as either "high" or "low." However, typical integrated circuits, or "chips," utilizing digital logic also include data signals at many voltage levels. As a result, sense amplifying circuits are utilized to interpret, or "separate" the data signals, driving them to a target high or low voltage level, thereby producing the correct voltage level for other digital circuits on the chip. For example, a capacitor-type memory cell for a dynamic random access memory ("DRAM") device utilizes very small bit signals. Sense amplifier circuits residing on the DRAM device separate the bit signals by comparing them to a precharge level, which is defined as a level between the high and low levels of the memory cell. If the level of the bit signal is above the precharge level, then the sense amplifier circuit interprets the signal as a high, or logic 1, signal and drives the bit signal to the target high voltage. If the level of the bit signal is below the precharge level, then the sense amplifier circuit interprets the signal as a low, or logic 0, signal and drives the bit signal to the target low voltage.
As technology progresses, the speed and accuracy of sense amplifier circuits become more critical. For this reason, chip designers have tried various methods to speed up the signal separation. For example, U.S. Pat. No. 5,257,232 to Dhong et al. teaches a method of overdriving a sense amplifier circuit to speed up the signal separation. Although this method succeeds in separating the signals quickly, it has drawbacks. One such drawback is that the overdriving is inaccurate because when the sensing is complete, the bit signals do not end up at the target high and low voltage levels. Therefore, during an equalization period, the bit signals do not return to the desired precharge level. Instead, the bit signals return to a level that is offset, due to the overshot voltage during the sensing. As a result, the precharge level becomes altered, and the sense amplifier circuits do not interpret the next bit signal correctly.
Therefore, what is needed is a method and apparatus for overdriving signals to achieve fast signal separation, while maintaining the accuracy of sense amplifier operation.